1. Field of the Invention
The present invention relates to a method for manufacturing a golf club, particularly to a material used in the method.
2. Description of the Prior Art
In a golf club head, one of means for enlarging the sweet area, i.e., an area on a face where a ball travels comparatively straight and well when struck thereon, is to large-size a club head itself. In recent years, some heads with more than 300 cc volume have come onto the market. Even if a club head is large-sized, yet the total weight thereof must be suppressed so as to ensure the easiness to handle. As a result, the head must be formed hollow inside, and its outer shells must be thinned. However, thinned outer shells generally lead to decreased strength of a club head, even to the likelihood of the head being damaged due for example to the impact force at the time of striking balls. As you need ensure sufficient strength of a face for striking balls, a face has heretofore been formed to at least 3.0 mm thickness.
The main current of recent golf clubs has been directed to titanium or titanium alloy head. As titanium alloy is lighter but stronger than stainless steel, the degree of freedom in designing a head is increased, thereby generally enabling the manufacture of a club head which is larger than a head made of stainless steel. Consequently, the sweet area is enlarged, as mentioned above, so that the stable flight of golf balls can be obtained.
Conventional titanium alloy based materials for a golf club head have been alpha+beta type alloys such as Ti-6Al-4V alloys and Ti-4.5Al-3V-2Fe-2Mo alloys, or beta type alloys such as Ti-15V-3Cr-3Sn-3Al alloys, and etc. The Ti-6Al-4V alloys have been most frequently used among such titanium alloys in various industrial fields, which however, are not suitable for cold working, and thus a great deal of labor and costs have been required to form a plate to a 1 to 5 mm thickness, the dimension generally required for the materials of a golf club. Therefore, casting has been employed for manufacturing such Ti-6Al-4V alloy made head, as disclosed in Japanese Patent Un-Examined Publication No.3-230845. However, as titanium is an easily oxidizable metal, casing is not able to be carried out in the atmosphere. Further, titanium is easily reacted with a casting mold, and thus extremely high technology is required, thereby eventually leading to increased costs. In addition to the foregoing, castings have a drawback of resultant inferior strength, because they can not undergo tissue-control as compared to rolled materials. In contrast, ultra plasticity working is possible for the aforesaid Ti-4.5Al-3V-2Fe-2Mo alloys, and thus high strength is more easily obtained by the alloys than by castings. However, as even the plasticity working requires special installations, and is highly time-consuming, the method is not suitable for mass production. Under the above-mentioned circumstances, beta type alloys with better working ability, such as Ti-15Mo-5Zr-3Al and Ti-15V-3Cr-3Sn-3Al, have more often come to be used. These alloys are formed by cold working to a preset thickness, and then are subjected to thermal treatment for removing distortions caused by cold working and undergoing solution treatment, thereby assembling them into a golf club head.
For related prior art, Japanese Un-Examined Patent Publication No.9-59731 discloses a titanium alloy for golf club head, said titanium alloy containing: zirconium and oxygen by the amounts in the ranges satisfying the following inequalities: the weight percentage content of Zrxe2x89xa71.0(%), in which Zr+25O2xe2x89xa75(%), and 3 X Zr+220 X O2xe2x89xa686(%); a slight amount of at least one selected from among aluminum, tin, copper, and chromium; and titanium and inevitable impurities as the remaining components. This prior art also teaches the manufacture of a face of a golf club head by subjecting such titanium alloy to final rolling to effect 30% or more reduction in area, at 700xc2x0 C. or below. In the prior art, such titanium alloy is referred to as alpha type titanium alloy. In fact, Zr is not a beta stabilized element. Further, the prior art publication refers to the difficulty in cold working in a case where a beta stabilized element is added. Accordingly, the art disclosed by this prior art publication is not intended for beta type titanium alloy.
On the other hand, Japanese Un-Examined Patent Publication No.11-19255 describes that the face member and head body member of a golf club head are each formed from beta-type titanium alloy. The prior art publication discloses a method for manufacturing a golf club head such that the face member is subjected to cold or hot forging to become plastically deformed to have a predetermined configuration. This prior art publication also describes that the face member can have a thickness of about 2.7 mm at the central portion thereof. Although this prior art teaches the use of cold or hot forging as a method of working the face member of a golf club head, it is silent with any advantage such as the improvement of durability to be resulted therefrom, only describing that any suitable methods may be selectively used for forging, irrespective of either cold or hot forging, and thus, it only refers to the types of forging as examples. In fact, cold working is generally subjected to a large deformation resistance, resulting in inferior workability, so that it is difficult to carry out. Accordingly, it is unlikely for those skilled in the art to select cold working. Further, even though the techniques disclosed by the prior art is used to cold work a beta type titanium alloy, yet it is not possible to manufacture a good club head. In other words, whilst a titanium alloy has extremely high specific tensile strength (strength/density) and corrosion resistance among practical metallic materials, and thus it has higher specific tensile strength and corrosion resistance than steel materials such as S45C, yet it has a drawback that due to its poor cold working ability, the cold working of a beta type titanium alloy is not a suitable method for manufacturing a face of a golf club head.
Whereas, Japanese Un-Examined Patent Publication No.9-215786 discloses that the face member of a golf club head is formed from a beta type titanium alloy. Specifically, this prior art publication discloses that when manufacturing the face member, a shoulder round bar is heated up to a temperature range for hot working and then die-forged to obtain a desired shape thereof, which is then subjected to direct aging treatment without solution treatment, to thereby form the face member having desired properties including desired strength. This prior art further describes that as the solution treatment is omitted after the hot die-forging, the production process is simplified, and the strength is enhanced due to the synergy of work hardening and age hardening. However, even if the material is subjected to direct aging treatment after the hot die-forging process, the strength thereof is still too small for the material to be used for that of the face member that is required to have the smallest possible thickness, and thus it is inevitably cracked due to the impact at the time of striking balls.
Also, Japanese Un-Examined Patent Publication No.10-71219 discloses a forging step in which a titanium-made material bar is heated to hot forging temperature and die-forged to form the face member, and an aging treatment step for subjecting the same to aging treatment without solution treatment after the forging step. However, if the direct aging treatment is performed after the hot die-forging step, yet the same problem as mentioned in the foregoing paragraph occurs for the same reasons.
Also, Japanese Un-Examined Patent Publication No.5-70909 discloses a method of manufacturing an aluminum alloy pipe for use with hydraulic machines, in which an aluminum alloy ingot is hot extruded, drawn, and then subjected to aging treatment. Specifically, this prior art discloses that the reduction in cross sectional area after the drawing process should be 15 to 25%, relative to the cross sectional area after the hot extrusion process. This prior art publication teaches that the reason why the 15-25% reduction in cross sectional area by the drawing process, which, in other words, is 15-25% cold work reduction, is preferred is that in the case of the reduction of 15% or below, the introduction of cold strain is too little to obtain sufficient strength even through the aging treatment, while in the case of the reduction of 25% or above, lowered ductility is resulted thereby leading to lowered formability in bending process or pipe-enlarging process. However, it should be noted that the prior art includes no teachings nor suggestions concerning the manufacture of a golf club. Further, it is true that cold reduction is referred to therein, but a beta type titanium alloy has such a poor cold working ability in general that it is not suitable for the manufacture of the face member of a golf club head.
Also, in Japanese Un-Examined Patent Publication No.62-151551 is disclosed a method of manufacturing a Ti-15V-3Cr-3Sn-3Al titanium alloy for use as a cold worked material, in which the alloy is subjected to solution heat treatment by retaining it at 830 to 1150xc2x0 C. for 3 minutes to 5 hours and then cooling it at more than 18xc2x0 C./minute cooling rate, which is then cold worked to more than 50% cold reduction.
Specifically, this prior art publication teaches that the advantage of more than 50% cold rolling rate is in that with more than 50% rate, the crystal particle size of the re-crystallized material that was subjected to high temperature preliminary solution treatment, cold rolling and then solution treatment becomes smaller than that of the material that was subjected to low temperature preliminary solution treatment, cold rolling, and then solution treatment. The prior art also mentions solution aging treatment, and describes that the titanium or titanium alloy thus obtained is suitable as a material for a rocket ship, various kinds of chemical plants, desalination plant and the like, but it does not include any descriptions relating to golf club head. Further, cold working reduction is referred to therein, but a beta type titanium alloy generally has such a poor cold working ability that it is not suitable for the manufacture of the face member of a golf club head.
In recent years, however, a face has been required to be thinned still further in order to make a ball travel a still longer distance, which has caused a problem such that beta type alloy materials of golf club head which are manufactured by the above-mentioned conventional processes are not strong enough to prevent the cracks from developing due to the impact force at the time of striking balls. The cracks are presumably due to a face being too much thinned to a thickness of less than 3 mm when only a face of 3 mm or above could withstand the impact.
To eliminate the above-mentioned problems, it is, therefore, a primary object of the present invention to provide a method for manufacturing a golf club with excellent durability.
To attain the above object, there is proposed, from a first aspect of the invention, a method for manufacturing a golf club having a head, which comprises the steps of forming a beta type titanium alloy material into a plate material of a preset thickness by cold rolling so that cold rolling reduction may be 15% or above. Specifically, as cold rolling is employed as cold working, the forming of a beta type titanium alloy to a thin thickness by cold rolling leads directly to the forming of the face member close to a preset thin thickness. Thus, if a beta type titanium alloy generally has a poor cold working ability, yet it is possible to easily form the face member to a preset thin thickness, such as 3 mm or below, by cold rolling the same. Further, due to the cold rolling, deformation speed can be lowered, thereby making cracks less likely to occur. Furthermore, with such enhanced cold rolling reduction of 15% or above, the work-hardening caused by the cold working is allowed to remain, thus improving the durability and surface hardness of the material.
From a second aspect of the invention, there is also proposed a method for manufacturing a golf club, further comprising the step of subjecting the face member to direct aging treatment. With such aging, the durability and surface hardness of the material can be improved. Further, by subjecting the cold rolled material to direct aging without solution heat treatment, there is no longer need of such a long heat treatment time to obtain a sufficient hardness, due to work hardening by cold rolling as well as aging precipitation that proceeds more rapidly, whereby manufacturing costs can be suppressed.
From a further aspect of the invention, there is proposed a method for manufacturing a golf club, further comprising the steps of hot rolling a beta type titanium alloy material into a beta single phase by control of temperature and then forming the material into a plate material of a preset thickness by cold rolling.
Whilst a beta type alloy is an alloy of so-called precipitation hardening type to allow the precipitation of an alpha phase in a beta phase by aging treatment to thereby obtain hardness, the direct aging treatment without releasing distortions or strains caused by working a head structuring member through cold rolling and pressing, can facilitate the precipitation of an alpha phase, without removing the effect of work hardening generated by cold working so as to leave the same effect at such a low temperature needed for aging, whereby a high strength can be obtained in a short time. Furthermore, as such beta single phase has neither a two-layer tissue nor a fragile tissue, no cracks will be developed at the time of cold or hot press working.